A Mystical Skyfire Diamond is a meta gem with a 15% chance to grant 6 seconds of 320 Spell Haste Rating, with a 45 second internal cooldown. The buff given is called focus. The gem can proc off instant casts, but they will not consume focus.

Patch 2.4.0 (25-Mar-2008): An undocumented change to this gem. Now the 1 ppm "Focus" buff lasts for 4 seconds and Increases Spell Haste Rating by 320. The buff still has a 45sec internal Cooldown, but is no longer consumed. When combined with general lag, this benefit is not noticeable.Patch 2.3.0 (13-Nov-2007): A cooldown has been added to this item's effect, but the chance for it to trigger has been increased.Patch 2.1.0 (22-May-2007): The triggering rate on this item has been increased, but it now only reduces the cast time on spells by half; and Warlocks casting "Corruption" with maximum ranks in "Improved Corruption" will no longer consume the "Focus" buff from this gem.